- #445 highlighted the lack of good documentation around Arti's
  multiple runtime support, as well as it being difficult to determine
  what runtime was actually in use.
- Improve the documentation to solve the first problem.
- To solve the second problem, make Runtime require Debug (which is
  arguably a good idea anyway, since it makes them easier to embed in
  things), and print out the current runtime's Debug information when
  arti is invoked with `--version`.
- (It also prints out other Cargo features, too!)

fixes #445